1. Field of the Invention
The present invention relates generally to the field of positive displacement (PD) type blowers, compressors, and more specifically relates to a shunt pulsation trap for reducing gas pulsations and vibration, noise and harshness (NVH) and improving compressor off-design efficiency without using a traditional serial pulsation dampener or a sliding valve.
2. Description of the Prior Art
PD compressors are capable of generating high pressures for a wide range of flows and are widely used in various applications, for examples, as in pipeline transport of purified natural gas from the production site to consumers thousands of miles away; or in petroleum refineries, natural gas processing plants, petrochemical plants, and similar large industrial plants for compressing intermediate and end product gases; or in refrigeration and air conditioner equipment to move heat from one place to another in refrigerant cycles; or in many various industrial, manufacturing processes to power all types of pneumatic tools, etc.
A positive displacement compressor converts shaft energy into velocity and pressure of a gas media (in a broader sense it includes different gases or liquid and gas mixture) by trapping a fixed amount of gas into a cavity then compressing that cavity and discharging into the outlet pipe. A positive displacement compressor can be further classified according to the mechanism used to move the gas as rotary type, such as screw or scroll, and reciprocating type, for example like piston or diaphragm, as shown in FIG. 2a. Though each type of PD compressor has its own unique shape, movements, principle and pros and cons, they all have in common a suction port, a volume changing cavity and a discharge port where a valve controls the timing of the release of gas media. Moreover, they are all cyclic in nature and possess the same process cycle for the processed gas, that is, suction, compression and discharge. FIG. 3a-3b show the compression cycle of a conventional positive displacement compressor and FIG. 3c shows the generic structure of a cavity and discharge port connected to a serial outlet dampener. Gas flows into the compressor as the cavity on the suction side expands and traps the media that is then being compressed by a drive device (say a reciprocating piston or rotary lobe) as the trapped cavity volume is reduced. After a desired compression ratio or volume reduction ratio is reached, the discharge valve or porting is opened and gas flows out of the discharge into the outlet. The inlet volume is constant given each cycle of operation and discharge volume varies according to the compression ratio as designed. In a dry running positive displacement compressor, gas is compressed as dry media, while in an oil-flooded positive displacement compressor, lubricating oil is injected into the cavity that helps to lubricate and seal the gap and cool the gas at the same time.
Since PD compressor divides the incoming gas mechanically into parcels of cavity size for delivery to the discharge, it inherently generates pulsations with cavity passing frequency at discharge, and the pulsation amplitudes are especially significant under high operating pressures or off-design conditions of either under-compression or over-compression. An under-compression happens when the pressure at the discharge opening (system back pressure) is greater than the pressure of the compressed gas within the cavity just before the opening. This results in a rapid backflow of the gas into the cavity, a pulsed flow, according to the conventional theory. All fixed pressure ratio compressors suffer from under-compression due to varying system pressures. An extreme case is the Roots type blower where there is no internal compression at all, or under-compression is 100% so that pulsation constantly exists and pulsation magnitude is directly proportional to pressure rise from blower inlet to outlet. On the other hand, an over-compression takes place when pressure at discharge opening is smaller than pressure of inside the cavity, causing a rapid forward flow of the gas into the discharge. For most applications where the system back pressure is normally not a constant, a fixed pressure ratio PD compressor will result in either an under-compression or over-compression. This pressure difference is responsible for generating large amplitude pulsations that is common for all types of PD compressors. The gas pulsations generated by discharge pressure difference are generally within the gas discharge flow (called gas borne) and periodic in nature. They travel throughout the downstream piping system and if left uncontrolled, could potentially damage pipe lines and equipments, and excite severe vibrations and noises.
To control pulsations, a large dampener, usually in the form of sudden area change plenums consisting of a number of chokes and volumes, is required at the discharge and connected in series with the discharge port. It is fairly effective in pulsation control with a reduction of 20-40 dB, but it itself is large in size which creates other problems like inducing more noises due to additional vibrating surfaces, or sometimes induces dampener structure fatigue failures that could result in catastrophic damages to downstream components and equipments. At the same time, discharge dampeners used today create high pressure losses that contribute to poor compressor overall efficiency. Moreover, at the off-design conditions, say either an under-compression or an over-compression, compressor efficiency suffers more. The traditional method is to use a variable geometry design so that internal volume ratio or compression ratio can be adjusted to meet different system pressure requirements. These systems typically are very complicated structurally with high cost and low reliability. For this reason, PD compressors are often cited unfavorably with high pulsations, high NVH and low off-design efficiency when compared with dynamic types like the centrifugal compressor. At the same time, the ever stringent NVH regulations from the government and growing public awareness of the comfort level in residential and office applications have given rise to the urgent need for quieter and more efficient PD compressors.
The present invention is trying to meet these environmental protection and market needs to tackle the problem by a new approach by postulating a new pulsation theory that a combination of large amplitude waves and induced flow are the primary cause of gas-borne pulsations. The new theory is based on a well studied physical phenomenon as occurs in a shock tube (invented in 1899) where a diaphragm separating a region of high-pressure gas from a region of low-pressure gas inside a closed tube. As shown in FIG. 1a-1b, when the diaphragm is suddenly broken, a series of expansion waves is generated propagating from low-pressure to high-pressure region at the speed of sound, and simultaneously a series of pressure waves which can quickly coalesces (fully developed) into a shockwave is propagating from high-pressure to low-pressure region at a speed faster than the speed of sound, inducing rapid fluid flow behind the wave front at the same time. An interface, also referred to the contact surface that separates low and high pressure gases, follows at the same fluid velocity after the pressure or shock wave. By analogy, the sudden opening of the diaphragm separating high and low pressure is just like the sudden opening of compression cell to discharge gas at off-design conditions.
To understand the pulsation generation mechanism in light of the shock tube theory, let's review a cycle of a classical positive displacement compressor as illustrated in FIG. 3a-3d by following one flow cavity. Low pressure gas first enters the cavity formed by a casing and a drive device at compressor inlet as in the Suction Phase. Then the cavity is closed to the inlet and the trapped gas is being compressed as the drive device forces the trapped volume to decrease in the Compression Phase. When a desired compression ratio is reached, the cavity is suddenly opened to the outlet and discharged. A serially connected discharge dampener is there to attenuate pulsations generated in gas stream.
If the cavity pressure is less than the outlet pressure as in case of an under-compression, a backflow would rush into the cavity to equalize pressure inside as soon as the cavity is opened to the discharge, according to the conventional theory. Since it is almost instantaneous and there is no volume change taking place inside the cavity, the compression is regarded as a constant volume process, or iso-choric. However, according to the shock tube theory, the cavity opening phase as shown in
FIG. 3C resembling the diaphragm bursting of a shock tube as shown in FIG. 1b would generate a series of pressure waves or a shock wave into the cavity. The pressure or shock wave front sweeps through the low pressure gas inside the cavity and compresses it at a speed faster than the speed of sound as in case of the under-compression. For the case of over-compression, a fan of expansion waves would sweep through the high pressure gas inside the cavity and expand it at the same time at the speed of sound. This results in an almost instantaneous adiabatic wave compression or expansion well before the induced flow interface (backflow as in conventional theory) could arrive because wave travels much faster than the fluid. In this view, the waves are the primary driver for pressure equalization process for conditions of either under-compression or over-compression while the pulsating flow movement is simply the induced flow behind the pressure waves.
In view of the new theory to explain the pulsation generation in case of an under-compression, as the pressure or shockwave travels to low pressure cavity as shown in FIG. 3c, a simultaneously generated expansion wave front travels in the opposite direction causing rapid pressure reduction and inducing backflow down-stream. While for the case of an over-compression, as the expansion wave travels to high pressure cavity as shown in FIG. 3d, a simultaneously generated pressure or shock wave front travels in the opposite direction causing rapid pressure increase and inducing forward flow down-stream. This pressure wave front travelling downstream at a speed faster than the speed of sound and inducing a fast flow behind it is the dominant source of gas-borne pulsations for a positive displacement compressor. Any effective pulsation control should target this high speed large amplitude mixture of waves and induced flow while minimizing the main flow losses at the same time.
Since the amplitude of industrial gas pulsations is typically much higher than the upper limit of 140 dB of the classical theory of Acoustics, the small disturbance assumption and linearized wave equation cannot be used reliably anymore. Instead, the following rules based on the above discussed Shock Tube theory can be used in interim to determine the source of gas pulsation generation and to quantitatively predict its amplitude and travel directions. In principle, these rules are applicable to gas pulsations generated by any positive displacement fluid machines such as engines, expanders, or pressure compressors and vacuum pumps.
1. Rule I: For two closed compartments (either moving or stationery) with different gas pressure p3 and p1 (FIG. 1a), there will be no gas pulsation generated if the two compartments are kept separate;
2. Rule II: If the divider between high pressure p3 and low pressure p1 is suddenly removed, it will trigger gas pulsation generation at the opening as a mixture of large amplitude Pressure Waves (PW) or shock wave*, Expansion Waves (EW)* and an Induced Fluid. Flow (IFF)* with magnitudes as follows: *It can be demonstrated by Shock Tube theory that pressure waves and expansion waves have about the same pressure ratio, if both media are the same gas type (P2/P1)==(P3/P1)1/2, see Reference: Anderson, J., 1982, “Modern Compressible Flow”, McGraw-Hill Book Company. New YorkPW=p2−p1  (1)EW=p3−p2  (2)ΔU=(p2−p1)/(d1×W)  (3)where d1 is the gas density, W the speed of shock wave travelling into the low pressure region andp2=(p3×p1)1/2   (4)
3. Rule III: the generated Pressure Waves (PW) or shock wave travel at the speed of shock wave W low pressure region while Expansion Waves (EW) move at the speed of sound in the direction opposite to PW, while at the same time both waves induce an unidirectional fluid flow (IFF) moving in the same direction as the pressure waves (PW).
Pay attention to Rules II which gives the location of gas pulsation source as the place of sudden opening between p3 and p1. It also indicates the sufficient conditions for gas pulsation generation: the existence of both pressure, difference and sudden opening. Because all PD fluid machines convert energy between shaft and fluid by dividing incoming continuous fluid flow into parcels of cavity size for delivery to discharge as indicated by its cycle, there is always a “sudden” opening at discharge to return these discrete parcels of cavity size back to a continuous stream again. So the two sufficient conditions are satisfied at the moment of discharge opening if there is a pressure difference existing between the cavity and outlet it is opened to. For compressors operating at off-design points with a fixed internal compression ratio, it is either an over-compression or under-compression as described previously. At design point, there will be no pressure difference induced pulsation according to the above Rule II. Since Roots type has no internal compression, it is always a case of under compression and is inherently generating gas pulsation. The pulsation magnitude predicted by Rule II can be very high if (p3-p1) is large for an un-throttled (or infinitely fast) opening as in a shock tube. However, most PD type fluid machines operate with finite discharge opening speed which throttles the induced fluid flow to a maximum sonic velocity that takes place at a pressure ratio of 1.89. In addition, a suddenly moved hardware (like lobe, valve disk) induced flow pulsations co-exist with pressure difference induced pulsation, but its magnitude is typically much smaller for most industrial PD type fluid machinery. FIG. 2b shows graphically the above relationship between the initial unbalanced pressures and the amplitude of the resulting gas pulsations generated.
It should also be pointed out the drastic magnitude and behavior difference between acoustic waves and gas pulsations discussed above. First of all, the acoustics is limited to pressure fluctuations below level of 140 dB, equivalent to pressure 0.002 Bar or 0.03 psi. For industrial fluid machinery, the measured gas pulsations that are typically in range of 0.3-30 psi (or even higher), or equivalent to 160-200 dB. So gas pulsation pressures are much higher and well beyond the pressure range for acoustics. Physically, the acoustics are sound waves travelling at the speed of sound with no macro fluid movement with it while gas pulsations are a mixture of strong pressure and expansion waves travelling in opposite directions that also induce an equally strong macro fluid flow travelling unidirectionally with speeds from a few centimeters per second up to 1.89 times of the speed of sound (Mach Number=1.89). It is this large pressure difference and potentially huge force that could directly damage system and components on its travelling path, in addition to exciting vibrations and noises. With the above Gas Pulsation Rules, it is hoped that more realistic gas pulsation calculation is possible and the true nature of gas pulsations can be realized and fully appreciated.
Accordingly, it is always desirable to provide a new design and construction of a positive displacement compressor that is capable of achieving high gas pulsation and NVH reduction at source and improving compressor off-design efficiency without using a traditional serial pulsation dampener and a variable geometry while being kept light in mass, compact in size and suitable for high efficiency, variable pressure ratio applications at the same time.